I'm trying to make a decision on a few different subs, and I have a question on the HSU subs. Could someone tell me in "newbie" terms the difference in the VTF-1 and the STF-2 subs? I see on the website that the VTF-1 is variable tuning. I have no idea what that means. Is that something that's worth paying the extra money for? I have a 21'x13'x8' room and it will be used 95% movies/TV and 5% music/games. I will also be using a Denon 1909 receiver. I haven't decided on the remaining speakers yet.